Processing signals of various systems include channel equalization. The channel equalization is used to compensate a fading effect of a multipath channel which becomes a fundamental problem in communication systems.
A variety of channel equalization techniques have been developed for the use with single carrier transmission systems according to the related art and the latest Code Division Multiple Access (CDMA) systems. An interest in multi carrier transmission techniques in which dedicated channel equalization techniques are to be used has been higher as a result of an increase in data rates and signal bandwidths in new future systems. In the multi carrier transmission system, a transmitted high rate data stream is divided into a plurality of low rate sub-channels which are partially overlapped in a frequency domain.
For multiplexing and demultiplexing of the sub-channels, various techniques have been used. For example, Orthogonal Frequency Division Multiplexing (OFDM) techniques and Filter Bank based Multicarrier (FBMC) techniques have been known. The FBMC techniques may be referred to as Discrete Wavelet Multitone (DWMT) techniques.
In the OFDM system and Discrete Multitone (DMT) system which is a baseband version thereof, the high rate data stream is divided into the plurality of the low rate stream simultaneously transmitted through a plurality of sub-carriers in order to reduce dispersion relative to a time due to multipath delay spread. The sub-channels are multiplexed and demultiplexed through a pair of Inverse Fast Fourier Transform (IFFT)-Fast Fourier Transform (FFT) operations. In OFDM and DMT systems, a time domain guard interval and simple 1-tap frequency domain equalization applied to all OFDM symbols are generally used for the channel equalization. In a guard time, the OFDM symbol is cyclicly extended so as to avoid inter-carrier interference.
The OFDM and DMT systems are very steady in a channel equalization viewpoint. Meanwhile, as described below, a study on the FBMC system has been actively performed because of an advantage which is obtained by the FBMC system.
The FBMC system according to the related art includes a transmitting and receiving method based on a Polyphase Network (PPN) structure in a time axis after the IFFT and a transmitting and receiving method based on a frequency spreader and an overlap/sum structure in a frequency axis before the IFFT.
The transmitting and receiving method based on the PPN structure implements a filtering using a convolution operation of the time axis to a sum of a weighted sum of M lengths by utilizing the PPN and then implements an offset-Quadrature Amplitude Modulation (QAM) by applying two PPN modules through a time difference. An equalizer is used in a time axis in the receiver by performing a filtering of the time axis.
The transmitting and receiving method based on the frequency spreader and overlap/sum structure is performed in the transceiver of the FBMC system as illustrated in FIGS. 1 to 3.
FIG. 1 illustrates a configuration of a transmitter in a FBMC system according to the related art. FIG. 2 illustrates a configuration of a receiver of an FBMC system according to the related art. FIG. 3 illustrates a detailed configuration of a transmitter according to the related art.
Referring to FIG. 1, the transmitter in the FBMC system according to the related art performs IFFT of a (K×M) length after performing filtering by an oversampling and prototype filter in the frequency axis before the IFFT and then performs a repeated transmission using an adder and a memory.
Referring to FIG. 2, the receiver in the FBMC system according to the related art performs a frequency axis filtering through a frequency de-spreader so as to increase a size of FFT by (K×M). A frequency axis one-tap equalizer is performed in the receiver by performing the filtering in the frequency axis.
Referring to FIG. 3, the receiver in the FBMC system according to the related art is configured with an IFFT unit 10 and a Parallel-Serial (P/S) conversion unit 20.
The IFFT unit 10 receives an input of a plurality of data. One piece of data di(mM) among the plurality of pieces data is spread into KM pieces of data before the IFFT unit 10 and is referred to as “weighted frequency spreading”. The data di(mM) spread into KM pieces of data is multiplied by (2K−1) number of frequency axis filter coefficients. The data di(mM) multiplied by the (2K−1) number of frequency axis filter coefficients is inverse fast Fourier transformed by the IFFT unit 10. The P/S conversion unit 20 receives an input of each data inverse fast Fourier transformed in parallel and outputs the data in series.
Consequentially, the size of the IFFT increases by a multiple of prototype filter order K so as to increase a complexity. The structure of the transmitter is represented to be symmetrical in the receiver so that a size of FFT increases by a multiple of K and a complexity increases in the receiver.
The PPN based transmitting and receiving structure has an advantage in that sizes of IFFT and FFT are maintained as M, but has a disadvantage in that an equalization process is to be performed in the time axis in the receiver due to the filtering in the time axis.
Further, the frequency spreader and overlap/sum structure based transmitting and receiving structure has an advantage in that the one-tap frequency axis equalizer can be used in the receiver by the filtering in the frequency axis, but has a disadvantage in that sizes of IFFT and FFT increase by K-fold so as to increase a complexity.
Therefore, both the PPN structure based transmitting and receiving structure and frequency spreader and overlap/sum structure based transmitting and receiving structure use IFFT and FFT of a conventional M size (or length) and is difficult to interwork with the OFDM system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.